Turbomachinery, such as turbocompressors and turboexpanders, are used to process gas from a first pressure to a second pressure and in the process the gas experiences a significant change in temperature. For example, in a turboexpander such as is often used in a cryogenic air separation plant, gas at a high pressure is passed through the turboexpander and expanded to a low pressure, thus experiencing a reduction in temperature and generating refrigeration for the cryogenic process. Examples of turbomachinery may be found in U.S. Pat. No. 4,430,011 - Kun and U.S. Pat. No. 4,472,107 - Chang et al.
Turbomachinery is characterized by a stationary outer housing and a rotatable shaft axially aligned within the housing. Mounted on the shaft are impeller blades through which the gas passes while experiencing the pressure change. The blades are often covered by a shroud for protection and improved performance. The assembly, i.e., shaft, blades and shroud, rotate at a very high speed, typically in excess of 5000 rpm when the turbomachinery is in operation. In order to avoid damage to turbomachinery parts, the rotating assembly within the housing is spaced from the stationary housing.
The space between the rotating assembly and the stationary housing is a source of inefficiency because it provides a pathway for gas to bypass the turbomachine. In order to reduce the amount of gas passing through this spacing, practitioners in this art typically place a seal within the spacing. Generally the seal is a labyrinth seal.
A labyrinth seal comprises a series of knife-like or tooth-like edges on the rotating part which are adjacent softer seal material on the stationary housing. The edges extend through the spacing between the rotating and stationary elements to nearly contact the stationary element. Thus there is created a series of extremely small annular spaces between the rotating and stationary parts making it very difficult for gas to pass through each of the serial small spaces in order to bypass the turbomachinery.
A common problem with labyrinth seals used in turbomachinery is the tendency of the spacing between the rotating edges and the seal material to change during operation of the turbomachinery due to temperature effects. Due to the significant temperature change of gas as it passes through the turbomachinery, there is a significant temperature difference between the rotating assembly, which experiences a temperature change in the direction of the gas temperature, and the stationary housing which is at the initial temperature. Thus the edges which are on the rotating assembly are at a different temperature than the seal material which is on the stationary housing. This temperature difference causes the two parts of the labyrinth seal to expand or contract at different rates and this differential thermal movement in the seal parts causes the space between them to change. The larger is the space between the seal parts the greater is the amount of gas which can bypass the turbomachinery and the greater is the efficiency loss experienced. However, spacing which is too close may result in unreliable operation if the parts come into contact.
It is therefore an object of this invention to provide a labyrinth seal for turbomachinery which experiences a reduced differential thermal movement between rotating and stationary seal parts.